Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. As used herein, pixels and sub-pixels are not distinguished and refer to a single light-emitting element. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode (LED) displays.
Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In a passive-matrix device, the substrate does not include any active electronic elements (e.g. transistors). An array of row electrodes and an orthogonal array of column electrodes in a separate layer are formed over the substrate; the intersections between the row and column electrodes form the electrodes of a light-emitting diode. External drivers then sequentially supply current to each row (or column) while the orthogonal column (or row) supplies a suitable voltage to illuminate each light-emitting diode in the row (or column).
In an active-matrix device, an active pixel circuit controls each pixel. Typically, each pixel circuit includes at least one transistor. For example, referring to FIG. 8, in a simple active-matrix organic light-emitting (OLED) display known in the prior art, each pixel 89 includes an optical element 15, e.g. an OLED emitter, controlled by a pixel circuit 80 that includes a selection circuit 801 and a driving circuit 802. The selection circuit 801 includes select transistor 81 for selecting pixel information, and a capacitor 84 for storing a charge specifying the desired luminance of the pixel. The driving circuit 802 includes a drive transistor 82 for providing current to optical element 15. Control of the optical element 15 is typically provided through a data signal line 85 and a select signal line 86.
Referring to FIG. 9, according to the prior art, an active-matrix display 90 includes a matrix 91 of pixels 89 arranged in rows and columns, each having a selection circuit 801 as described above. Each row has a respective select signal line (85a, 85b, 85c), and each column has a respective data signal line (86a, 86b, 86c). A gate driver 95 controls the select signal lines and source driver 96 controls data signal lines. Therefore, any failure in any select signal line 85 or data signal line 86 (e.g. as shown in FIG. 8), or a gate driver 95 or a source driver 96 providing signals on that line, causes malfunction of the pixels attached to that line. Data signal lines are commonly referred to as column lines, and select signal lines are commonly referred to as row lines, but those terms do not require any particular orientation of the panel. Furthermore, each selection circuit 801 is connected to a unique pair (data signal line 85, select signal line 86), and is addressed by that pair.
One common, prior-art method of forming active-matrix pixel circuits deposits thin films of semiconductor materials, such as silicon, onto a glass flat-panel substrate and then forms the semiconductor materials into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of displays employing such materials.
Employing an alternative control technique, Matsumura et al describe crystalline silicon substrates used for driving LCD displays in U.S. Patent Application Publication No. 2006/0055864. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. A matrix-addressing pixel control technique is taught.
Both the active-matrix and the passive-matrix control schemes rely on matrix addressing, the use of two control lines (e.g. 85, 86 in FIG. 8) for each pixel to select that pixel. This technique is used because other schemes such as direct addressing (for example as used in memory devices) require the use of address decoding circuitry that is very difficult to form on a conventional thin-film active-matrix back plane, and is impossible to form on a passive-matrix back-plane as such a back-plane lacks transistors. Another data communication scheme used e.g. in CCD image sensors as taught in U.S. Pat. No. 7,078,670, employs a parallel data shift from one row of sensors to another row, and eventually to a serial shift register that is used to output the data from each sensor element. This arrangement requires interconnections between every row of sensors and an additional, high-speed serial shift register. Moreover, the logic required to support such data shifting would require so much space in a conventional thin-film transistor active-matrix back-plane that the resolution of the device would be severely limited, and would be impossible in a passive-matrix back-plane, which lacks transistors.
U.S. Pat. No. 6,259,838 to Singh et al. teaches a display device employing a plurality of light-emitting elements disposed along the length of a light-emitting fiber, such as an optical fiber. This scheme provides a one-dimensional flow of information controlling OLED display elements arranged along the fiber. However, in high-resolution displays, this scheme requires precise positioning of a large number of fibers, e.g. one per row. Positioning errors can cause visible non-uniformity and reduce yields. Furthermore, any breaks in the fiber can deactivate all pixels after the break, or all pixels attached to the fiber.
Both matrix-addressed and serially shifted control schemes for display devices are vulnerable to interconnect failures. Typically, a single row or column connection failure results in an entire row or column fault. Such failures can occur in manufacturing or from use.
It is known to employ bi-directional level shifters to transmit signals having different voltage levels on two portions of a single bus. For example, U.S. Pat. No. 5,680,063 to Ludwig et al. describes such a circuit.
There is a need, therefore, for an improved apparatus for display devices that improves the tolerance of the display to wiring interconnection faults.